The main goal of this work is to develop an understanding in growing nerve fibers, (1) of the mechanism of plasmalemmal expansion, (2) of the process of maturation and regional differentiation of the plasma membrane, and (3) of the role of the growth cone plasmalemma in the mechanisms of guidance and recognition during nervous system development. The major approaches to be used involve binding studies with a battery of different lectins on cultured neurons and their rapidly growing processes. The lectins are used in ferritin-conjugated form for their detection and localization with the electron microscope, and the parameters of interest are: Lectin binding as a function of the state of maturation and of the type of nerve fiber; appearance of new lectin receptors, and their relative movement, during nerve fiber growth. This work is to be complemented with studies on the intracellular pathway and the site(s) of plasmalemmal insertion of radioactive putative membrane precursors utilizing pulse-chase experiments and radioautography at the electron microscope level. A further technique to be employed is freeze-fracture to investigate, as a function of time and maturation, the densities of intramembranous particles found in various areas of neuronal plasmalemma.